Lt. Gen. Olusegun Obasanjo, the slightly stocky, mustached leader of Nigerla's federal military government, is a soft-spoken man who has had a deep abiding interest in the plight of the oppressed black man in white-rule southern Africa ever since his early school days.

Today, the 41-year-old, British-trained engineering officer has gained a national mandate to make his personal concern that of the most populous nation in Africa and to impress it as a central issue in Nigeria's economic and political relations with its Western partners.

THis other great goal, according to his own declarations, is to make Nigeria, rich in oil and teaming with 80 million people, into a "great modern black nation."

Like President Carter, Obasanjo is a Baptist and the two leaders are scheduled to pray together here today. He is married with five children but little is known about his private life otherwise.

In asserting his country's role as a Third World leader, Obasanjo is said to regard himself as the faithful disciple of his slain colleague, Gen. Murtala Muhammed. It was he who first began to flex Nigeria's muscles in African politics by helping the present Angolan government to win its diplomatic and military struggle against the two pro-Western and South African-backed factions during the 1975-76 civil war there.

Obasanjo, who is playing host to the first official visit ever of an American president to this continent, came to power in the unsuccessful coup of Feb. 13, 1976 that took the life of his close friend General Muhammed. He had been serving in effect as "prime minister" under Muhammed running the day to day afairs of the federal government.

He was the unanimous choice of the supreme military council to replace Muhammed as the new head of the federal military government and commander in chief of all Nigerian armed forces.

A Yoruba from Western Nigeria, the general made a career out of the military beginning in 1958 and was sent abroad for training at the Mons Officers Cadet School in Britain. At that time, Nigeria was still a British colony.

Aa an engineer, Obasanjo attended various professional schools in Britain and India and won a citation as the best Commonwealth student ever to attend the British Royal College of Military Engineering.

Within the Nigerian army, he served under the British in the Cameroons before Nigeria's independence in 1960 and then in the U.N. peace-keeping force in the then Congo, today known as Zaire. By 1963, he was the commander of the Nigerian army's only engineering unit and then switched to become head of the 2nd Division based in Ibadan.

His fame as a soldier and commander was probably made when he was in charge of the 3rd Commando Division that took much of the brunt of the fighting against the IBO secessionist movement in the eastern region then known as Biafra. In fact it was he who acepted the surrender of the rebel forces at the end of the war in January 1970.

Since he took over from Muhammed, Obasanjo has pressed ahead with the federal government's promise to return Nigeria to civilian rule by October of next year.He has also sought to impose a measure of discipline on to impose a measure of discipline on Nigeria's free wheeling, American-style society and has sharply criticized his countrymen for their "mad rush for quick and easy money."

Obasanjo has little time for the ideological disputes over socialism and capitalism sweeping many African countries and sees a standstill prevailing generally in the rivalry between East and West.

"I am convinced beyond all doubt that the decision to choose between capitalism and socialism and the energy expended to making this choice becomes diversionary and wasted," he said in a speech at the opening of the new Nigerian Command and Staff College last September.

These ideologies, he said, as proclaimed by many African leaders are "mythical instruments of political oppression" opening the door as often as not to "political and economic morass."

Instead, he has set out to build what he calls a "disciplined, fair, just and humane African society" in Nigeria.